1-19The material through which sound waves travel is called the medium. The density of the mediumdetermines the ease, distance, and speed of sound transmission. The higher the density of the medium, theslower sound travels through it.The detector acts as the receiver of the sound wave. Because it does not surround the source of thesound wave, the detector absorbs only part of the energy from the wave and sometimes requires anamplifier to boost the weak signal.As an illustration of what happens if one of these three elements is not present, lets refer to ourexperiment in which a bell was placed in a jar containing a vacuum. You could see the bell being struck,but you could hear no sound because there was no medium to transmit sound from the bell to you. Nowlets look at another example in which the third element, the detector, is missing. You see a source (suchas an explosion) apparently producing a sound, and you know the medium (air) is present, but you are toofar away to hear the noise. Thus, as far as you are concerned, there is no detector and, therefore, no sound.We must assume, then, that sound can exist only when a source transmits sound through a medium, whichpasses it to a detector. Therefore, in the absence of any one of the basic elements (source, medium,detector) there can be NO sound.Q18.Sound waves transmitted from a source are sometimes weak when they reach the detector. Whatinstrument is needed to boost the weak signal?TERMS USED IN SOUND WAVESSound waves vary in length according to their frequency. A sound having a long wavelength is heardat a lowpitch (low frequency); one with a short wavelength is heard at a highpitch (high frequency). Acomplete wavelength is called a cycle. The distance from one point on a wave to the corresponding pointon the next wave is a wavelength. The number of cycles per second (hertz) is the frequency of the sound.The frequency of a sound wave is also the number of vibrations per second produced by the sound source.Q19.What are the three basic requirements for sound?CHARACTERISTICS OF SOUNDSound waves travel at great distances in a very short time, but as the distance increases the wavestend to spread out. As the sound waves spread out, their energy simultaneously spreads through anincreasingly larger area. Thus, the wave energy becomes weaker as the distance from the source isincreased.Sounds may be broadly classified into two general groups. One group is NOISE, which includessounds such as the pounding of a hammer or the slamming of a door. The other group is musical sounds,or TONES. The distinction between noise and tone is based on the regularity of the vibrations, the degreeof damping, and the ability of the ear to recognize components having a musical sequence. You can bestunderstand the physical difference between these kinds of sound by comparing the waveshape of amusical note, depicted in view A of figure 1-13, with the waveshape of noise, shown in view B. You cansee by the comparison of the two waveshapes, that noise makes a very irregular and haphazard curve anda musical note makes a uniform and regular curve.